In recent years, fiber-optic cables have been increasingly used for communications, particularly in telephone systems. Reasons for this increased usage include the facts that optical fibers are lighter in weight and less expensive than electrical conductors for the same information capacity, and are not subject to electrical interference. Typically, a communication system includes a light source such as a laser diode or an LED, and a photodetector such as a photodiode, connected through a single mode or multimode fiber-optic cable. Information is typically transmitted in digital form, as a series of light pulses that form a bit stream.
In order to increase the information-carrying capacity of a fiber-optic cable, frequency and time division multiplexing techniques have been widely explored. Examples of prior art frequency division multiplexing optical communication systems are described in U.S. Pat. Nos. 4,326,243 and 4,592,043. However, a number of problems have been encountered in implementing such systems. These problems include frequency variations of the semiconductor light sources, matching of the multiplexer and demultiplexer coupling frequencies, and the need for relatively large channel spacing to accommodate aging effects and manufacturing tolerances of semiconductor lasers.
Coherence multiplexing is a comparatively new technique for carrying multiple data channels on a single optical fiber or waveguide. In a coherence multiplexed system, the transmitted information is carried in the structure of the complex autocorrelation or self-coherence function of the transmitted optical signal. The advantages of coherence multiplexing are that it uses optical signal processing structures that are inherently simpler than those required for frequency division multiplexing, and uses electronic signal processing structures that are simpler than those generally associated with time division multiplexing.
Prior coherence multiplexed optical transmission systems have modulated information onto a carrier by passing the carrier through an unequal arm length Mach-Zehnder interferometer, and by using a modulation signal to vary the optical path length of one of the interferometer arms. Although this technique is effective, the requirement for electro-optic modulators requires the transmitter to be constructed in an electro-optic substrate. Development of a coherence multiplexed system that did not require electro-optic modulators would therefore allow the system to be fabricated in a wider variety of materials.